In recent years, developments of electric vehicles (EV), hybrid electric vehicles (HEV) and fuel cell vehicles (FCV) have been advanced against the background of escalating environmental protection movement. For a power source for driving motors used on those vehicles, a rechargeable secondary battery is suitable. In particular, what is attracting the attention is a non-aqueous electrolyte secondary battery such as a lithium-ion secondary battery expected to provide high capacity and high output.
A non-aqueous electrolyte secondary battery is provided to have a positive electrode active material layer that is formed on a surface of a current collector and includes a positive electrode active material (for example, LiCoO2, LiMO2, or LiNiO2). Additionally, the non-aqueous electrolyte secondary battery is provided to have a negative electrode active material layer that is formed on a surface of a current collector and includes a negative electrode active material (for example, metal lithium, carbonaceous materials such as cokes, natural and synthetic graphite, metal materials including Sn and Si and oxides of them).
A binder for binding an active material which is used for an active material layer is classified into an organic solvent-based binder (binder which is not dissolved/dispersed in water but dissolved/dispersed in an organic solvent) and an aqueous binder (a binder which is dissolved/dispersed in water). The organic solvent-based binder can be industrially disadvantageous due to high cost such as raw material cost for an organic solvent, recovery cost, and cost relating to waste processing. Meanwhile, the aqueous binder has an advantage of lowering a burden on environment and greatly suppressing an investment on facilities of a production line, since water as a raw material is conveniently available and only water vapor is generated during drying. The aqueous binder also has an advantage that, since the aqueous binder has a high binding effect even with a small amount compared to an organic solvent-based binder, it can increase a ratio of an active material per same volume so that a negative electrode with high capacity can be achieved.
From the viewpoint of having those advantages, various attempts have been made for forming a negative electrode by using an aqueous binder as a binder for forming an active material layer. For example, with regard to a technique of using sulfonated latex as a binder for a negative electrode active material layer, a technique of using a rubber-based binder such as styrene-butadiene rubber (SBR) as a sulfonated latex is disclosed in JP 2003-123765 A. According to JP 2003-123765 A, it is described that the charge characteristics of a battery at low temperature or charge and discharge cycle service life characteristics can be improved by having such constitution.
However, according to the studies by the inventors of the present invention, it was found that the battery performances (in particular, service life characteristics after long-term cycle) of a non-aqueous electrolyte secondary battery in which an aqueous binder such as SBR was used for forming a negative electrode active material layer was still insufficient. The inventors of the present invention found that it was due to the depletion of a liquid in the negative electrode active material layer containing an aqueous binder as caused by poor wettability of an electrolyte liquid, and as described in JP 2003-123765 A, the wettability could be improved by sulfonating the latex. However, it was found that, even when the wettability of the electrode active material layer of only the negative electrode active material layer was improved, a poor soaking property of an electrolyte liquid into a positive electrode active material layer caused insufficient cycle characteristics of a battery.